Polyaniline complex with fullerene C60

European Polymer Journal 36 (2000) 2321±2326

Polyaniline complex with fullerene C60 Irina Sapurina a, Maksim Mokeev a, Viktor Lavrentev a, Vladimir Zgonnik a, a c, Jaroslav Stejskal c,* Miroslava Trchov a b, Drahomõra Hlavat a

c

Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg 199004, Russian Federation b Faculty of Mathematics and Physics, Charles University, 180 00 Prague 8, Czech Republic Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic Received 6 December 1999; accepted 16 December 1999

Abstract Polyaniline±fullerene composites were prepared either by solid-state blending of both components (composite I) or by the introduction of fullerene during polymerization of aniline (composite II). Composite I had a lower conductivity (1:5 10ÿ7 S cmÿ1 ) than composite II (7:2 10ÿ5 S cmÿ1 ). Investigation of the composites by solid-state 13 C-NMR and FTIR spectroscopy indicated the interaction between polyaniline and fullerene. X-ray diractograms suggest that the mechanical blending of the components leads to a decrease in the size of fullerene crystallites. In composite II, the diractograms proved the formation of a polyaniline±fullerene complex comprising the structure corresponding to a doped polyaniline. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polyaniline; Fullerene; C60 ; Complex formation

1. Introduction The preparation of composites based on conjugated polymers and buckminsterfullerene (C60 ) has opened a large area of scienti®c and technological interest. The photoinduced electron transfer from semiconducting polymers (as donors) to C60 and its derivatives (as acceptors) has been demonstrated in blends as well as in heterostructures prepared from these two materials. The latter interpenetrating phase-separated donor±acceptor composites appear to be ideal photovoltaic materials for the preparation of various types of optical devices such as switches, dynamic memory units, etc. Devices based on polythiophene derivatives show a photoresponse, which is competitive with UV-sensitized silicon photodiodes [1,2]. Zakhidov et al. [3] and Araki et al. [4] have reported that doping the composites of conjugated polymer and C60 with alkali-metal atoms results in a

*

Corresponding author. Tel.: +42-02-2040-3351; fax: +4202-36-79-81. E-mail address: [email protected] (J. Stejskal).

granular superconductivity of the C60 component embedded in the polymer matrix. There is an indication of increasing the thermal stability of conjugated systems by the introduction of C60 and its derivatives [5,6]. Watersoluble complex of C60 with poly(N-vinylpyrrolidone) was demonstrated to have a high antivirus activity [7]. The list of applications is far from being complete. Composites of several conducting polymers, viz. polyvinylcarbazole [8], polyalkylthiophene [9], poly(pphenylenevinylene) [10,11], have been studied during past years. The charge transfer between the C60 and the donor fragments leads to the doping of polymers, thus increasing their conductivity by making them p-type semiconductors. The doping level re¯ects the extent of interaction between the components. Among the conducting polymers, polyaniline (PANI) has attracted much attention because of its multiple electronic states, high conductivity that occurs upon doping, as well as for its easy and economic preparation, good environmental stability, and high application potential [12]. Wei et al. [13] were the ®rst to report the doping of PANI with C60 . Various authors have, however, noted the low doping degree of this polymer

0014-3057/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 0 1 2 - 4

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I. Sapurina et al. / European Polymer Journal 36 (2000) 2321±2326

[13±15]. In all the cases, the solution blending has been employed for preparation of PANI±C60 composites. Polyaniline base was dissolved in N-methylpyrrolidone (NMP) and C60 in chlorobenzene or toluene. The solutions were mixed and evaporated to produce ®lms of PANI±C60 composite ready for further research. It is known, however, that C60 reacts with both primary and secondary amines [16]. Consequently, there is a chance that NMP, used as a solvent, has not performed as an inert medium. It could competitively react with C60 and this might be a reason for the low doping degree in PANI. In the present work, two new ways of PANI±C60 composite preparation are investigated. These are based on the solid-state blending of the components and on the introduction of C60 into the reaction mixture used for the synthesis of PANI.

2. Experimental 2.1. Materials Aniline (Fluka, Switzerland) was puri®ed by double distillation at reduced pressure. Ammonium peroxodisulfate and toluene-4-sulfonic acid monohydrate (TSA) (Fluka, Switzerland), were used as delivered. Polyaniline (emeraldine) hydrochloride was prepared by oxidation of aniline with ammonium peroxodisulfate and deprotonated to PANI base with ammonium hydroxide [17]. Buckminsterfullerene C60 (>98%) puri®ed by HPLC was provided by the Ioe Physico-Technical Institute, Russian Academy of Sciences (St. Petersburg, Russian Federation).

2.2. PANI±C60 composites In the ®rst approach, equal weight amounts of PANI base and C60 were mechanically blended with a pestle in an agate mortar. The mixture was then heated for 1 h at 85°C [17]. In the second method, C60 was added during the polymerization of aniline. Fullerene (10 wt.% relative to aniline) was injected into the mixture containing aniline (0.4 M), dissolved in 3 wt.% aqueous solution of TSA. Polymerization was started by the addition of an aqueous solution of ammonium peroxodisulfate in an amount equimolar to that of aniline. The mixture was stirred at room temperature for 5 h. After polymerization, TSA was neutralized and repeatedly washed with excess of 1 M aqueous ammonium hydroxide. This procedure excludes PANI doping with TSA or other acids. The precipitate was separated by centrifugation, washed with acetone, and dried in vacuo at 25°C.

2.3. High-resolution solid-state

13

C-NMR

Spectra were obtained with a CPX-100 spectrometer (Bruker) at 25.18 MHz with a magic-angle spinning (MAS) technique at a rotation frequency of 3.5 kHz. All chemical shifts are given in ppm relative to tetramethylsilane. 2.4. Fourier-transform infrared spectra Spectra in the range 400±4000 cmÿ1 were recorded with a fully computerized Nicolet Impact 400 instrument (200 scans per spectrum at 2 cmÿ1 resolution). The samples were dispersed in potassium bromide and compressed into pellets. 2.5. Wide-angle X-ray diractograms Diractograms were recorded with an automated HZG 4A goniometer in the range of scattering angles 2h 4±40°. CuKa radiation was monochromatized with a nickel ®lter and a pulse-height analyzer and registered with a scintillation counter. 2.6. Conductivity The composites were pressed into a pellet with a manual hydraulic press at 700 MPa in vacuo. The DC electrical conductivity was measured by the two-probe method with the help of a cell with copper electrodes. 3. Results and discussion 3.1. Preparation of PANI±C60 composites The formation of PANI±C60 complex faces the problem to disperse both the components to a molecular level. The solubility of both the substances is limited. The only generally used solvent for PANI base, NMP, contains amino groups, which interact with C60 . We have observed slow irreversible changes in the visible spectrum of C60 dissolved in NMP. The competitive reaction of C60 with NMP may therefore complicate the doping interaction of C60 with PANI base. That is why another method of PANI±C60 complex formation has been sought. The ®rst way of PANI±C60 composite preparation (denoted as composite I) was based on the blending of equal masses of both the components followed by heat treatment. Such a ``dry'' solid-state blending in the absence of any solvent is simple and surprisingly ecient [17,18]. It has been successfully used to achieve doping of PANI base with hydroquinone [19] and with dodecylbenzenesulfonic [20], camphorsulfonic and picric acids [17,18]. The degree of PANI doping was high and, in


the case of picric acid, the conductivity of the composite increased from 10ÿ9 up to 10ÿ1 S cmÿ1 proving that the reaction between the components had taken place [17]. The second method used the aniline polymerization for introduction of C60 onto the growing PANI chains. Fullerene C60 was added to an aqueous solution of aniline, containing an oxidant and TSA, used to provide acidic reaction conditions. Here, the doping of PANI with fullerene is competed by the protonation with TSA. After the polymerization, the acid was removed by neutralization with aqueous ammonium hydroxide, and thus, the C60 is the only doping component potentially present. The resulting material was denoted as composite II. We have anticipated that this method would be helpful in improving the interaction between the reagents. The previous research concerning oxidative polymerization of aniline has indicated that PANI chains may become attached by a strong adhesion to a variety of substrates. MacDiarmid and Epstein [21] proposed that the reactive intermediate, possibly an oligomeric cation radical, is at ®rst adsorbed on the substrate and then, subsequently, the polymerization proceeds. Indeed, the growth of PANI ®lms during polymerization on glass surfaces has been observed [21±23]. A similar technique has been used for the coating of polystyrene microspheres by an overlayer of a conducting polymer [24±26]. Thus, it is likely that the formation of PANI ®lm need not start only on planar surfaces or micrometer-sized spherical objects, but also on substantially smaller objects represented by C60 and its microcrystallites. Thus, the resulting composite may have a structure of a C60 center containing strongly adsorbed PANI chains. Composites I and II obtained by two dierent approaches were studied by various methods. 3.2.

13

C-NMR spectra

13 C-NMR spectra of PANI (emeraldine) base and the corresponding form protonated (``doped'') with hydrochloric acid, PANI hydrochloride (Fig. 1), are shown in Fig. 2. The spectrum of PANI base consists of the main

Fig. 1. Polyaniline base reacts with an acid to yield protonated polyaniline.

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Fig. 2. 13 C-NMR spectra of (a) polyaniline base and (b) polyaniline hydrochloride.

signal at 121 ppm (Fig. 2a), assigned to the C±H groups of quinoid rings, and shoulders at 136 ppm, belonging to the benzenoid C±N carbons, and at 156 ppm of C@N quinoid origin. The spectrum of the protonated PANI has a broad structure with a center at 129 ppm (Fig. 2b). This signal is known to be attributed to the benzenoid carbons [27]. The resolution of this spectrum is signi®cantly lower because of high paramagnetism of the protonated PANI. It is well known that 13 C-NMR solid-state spectrum of C60 contains a single peak at 143 ppm. This signal can easily be detected in the spectra of both the composites (Fig. 3a and b). It can be seen that the peaks of quinoidring carbons shift from 121 ppm (Fig. 2a) to 125 ppm (composite I, Fig. 3a) and to 127 ppm (composite II, Fig. 3b). This suggests that the electron density has been transferred from PANI to C60 . The PANI structure is transformed from the undoped state to the doped form and the degree of doping is more pronounced in

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Fig. 4. FTIR spectra of polyaniline±fullerene C60 composites and of its individual components (The spectrum of polyaniline hydrochloride has been included for comparison).

Fig. 3. 13 C-NMR spectra of (a) polyaniline±fullerene C60 composite I and (b) composite II recorded without and (c) with the cross-polarization technique.

composite II. The indirect proof of the electrondensity transfer has been obtained with the help of a cross-polarization experiment as the appearance of a slight signal of fullerene (141 ppm) in the spectrum of composite II (Fig. 3c). 3.3. Fourier-transform infrared spectra The FTIR spectrum of PANI base alone (Fig. 4) is in good agreement with the previously reported results [28± 32]. The main peaks at 1590 and 1500 cmÿ1 correspond to the quinone and benzene stretching ring deformation. The 1308 cmÿ1 band is assigned to the C±N stretch of secondary aromatic amine, whereas in the region of 1010±1170 cmÿ1 , the aromatic C±H in-plane bending modes are usually observed. In the region of 800±880 cmÿ1 , the out-of-plane deformation of C±H on 1,4-disubstituted ring are observed. The band at about 1630 cmÿ1 is most probably connected with the presence of water in the sample or in potassium bromide.

In the spectrum of PANI hydrochloride, the main peaks at 1590 cmÿ1 and 1500 cmÿ1 are redshifted to 1570 cmÿ1 and 1480 cmÿ1 . The band characteristic of the conducting protonated form is observed at about 1240 cmÿ1 . It has been interpreted as C±N stretching vibration in the polaron structure [30]. The 1165 cmÿ1 peak is assigned to a characteristic vibration mode of the quinone ring. The band at about 1610 cmÿ1 is attributed to the stretching of imine N@ring bond. The infrared spectrum of C60 contains four strong intramolecular modes at x1 526 cmÿ1 , x2 576 cmÿ1 , x3 1182 cmÿ1 and x4 1428 cmÿ1 [33]. In some cases, three of the modes, x1 , x2 , and x4 , show doping-induced shifts when solid C60 is used for such doping [34]. The vibrational states of the single-crystal complexes of fullerene with organic donors have also been studied by FTIR spectroscopy [35]. In the spectrum of PANI±C60 composite I, the 1590 cmÿ1 peak (stretching of the quinoid rings) shows a redshift to 1580 cmÿ1 , whereas the position of the benzenoid-ring absorption at 1500 cmÿ1 remains about the same. The 1240 cmÿ1 band characteristic of the conducting polaron structure C±N appears. The band in the region of 1610 cmÿ1 re¯ecting the N@ring stretching vibration is missing in comparison with the protonated PANI hydrochloride. The characteristic absorption bands of C60 are well observed in composite I, and they are in the same positions as in fullerene alone. The spectral features at 617 and 1400 cmÿ1 belong probably to the contamination by ammonium sulfate (Fig. 4). Polyaniline base used for the preparation of the composite was unfortunately dierent from that characterized alone; that is why similar traces of impurities were not detected in the spectrum of a pure PANI base (Fig. 4). FTIR studies of composite I indicate the presence of the doping reaction of PANI, as well as C60 incorporation. These results also prove that the charge transfer


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from the imine segments of PANI to C60 molecules occurred to yield a polaron in PANI chains. The spectra of composites I and II are similar. Besides the 1240 cmÿ1 band characteristic of the conducting polaron structure C±N , the characteristic vibrational mode of the quinone ring at about 1165 cmÿ1 is observed in composite II. This supports the concept of a stronger interaction between PANI and C60 based on the 13 C-NMR results. No characteristic absorption bands of C60 are, however, found in composite II; this implies that the content of fullerene in the composite is low, although its presence can be proved by more sensitive methods, such as 13 C-NMR and X-ray diraction discussed later. 3.4. Wide-angle X-ray diractograms X-ray diraction pro®les of composite I (Fig. 5) and composite II (Fig. 6), together with the diraction curves of both the protonated and non-protonated forms of PANI and also of a neat fullerene C60 , were recorded. Xray diraction pattern of C60 powder exhibits a number of strong re¯ections in the range of the scattering angles 2h 10±35°, corresponding to a cubic-lattice crystalline symmetry [36]. The other component of composite I, PANI base, is quite amorphous [37]. Blending of these two components does in¯uence neither the crystalline modi®cation of C60 nor the amorphous phase of PANI base (Fig. 5). However, some interactions between C60 and PANI base can be expected, as the fullerene crystalline re¯ections in the composite become wider in comparison with the neat fullerene. We suppose that blending of C60 with PANI base results in a decrease in the crystallite size of C60 . Some additional re¯ections at 2h 20:2°; 22:8°; 28:5°; 29:2° and a series of faint re¯ections in the region of 2h 33±40° can be attributed

Fig. 6. WAXS diractograms of (±±) polyaniline±fullerene C60 composite II, (- - -) fullerene C60 , and ( ) protonated polyaniline hydrochloride.

to the traces of ammonium sulfate [38]. Obviously, this salt was not completely washed out of PANI base before its blending with C60 (cf. also the FTIR spectra of composite I in Fig. 4). Quite a dierent situation exists in the case of composite II, where fullerene C60 was added during the polymerization of aniline. X-ray diraction pattern of this sample (Fig. 6) is composed of the diraction curve of a neat C60 and the diraction curve, which is identical with that of the protonated form of PANI [39]. In this case, fullerene can obviously act as a dopant of PANI, similar to an acid being able to react with a non-conducting PANI base to give conducting protonated PANI. 3.5. Conductivity Conductivity of the composite components is low, 4:3 10ÿ9 S cmÿ1 for PANI base [17] and